Color picture tubes used in computer terminals, personal computers and the like are required to have a good focus characteristic; that is to say, the spot size of an electron beam which collides against a fluorescent screen needs to be small. The spherical aberration of a main lens has a great influence on this focus characteristic. To decrease the spherical aberration, the effective diameter of the main lens needs to be increased. Various methods have heretofore been proposed to increase the effective diameter of the main lens. FIG. 1 shows an example of a conventional large-diameter electron gun, in which its main lens is formed between a focus electrode 4 and an anode electrode 6. A plate electrode 5 is formed in the inside of the focus electrode 4, and a plate electrode 7 is formed in the inside of the anode electrode 6. Holes through which to individually pass three electron beams are formed within the focus electrode 4 and the anode electrode 6 by the respective inner plate electrodes 5 and 7. A hole 4xe2x80x2 which is formed in the focus electrode 4 and a hole 6xe2x80x2 which is formed in the anode electrode 6 are holes common for three electron beams. Examples of this type of electron gun are disclosed in U.S. Pat. Nos. 5,146,133, 4,599,534 and 4,581,560. As another method of increasing a lens diameter, there is a method of arranging an intermediate electrode between an anode electrode and a focus electrode to reduce the potential gradient in the main lens, thereby enlarging the effective lens diameter.
An example of this method is shown in FIG. 2. Examples of this type of electron gun are disclosed in Japanese Patent Laid-Open Nos. 180648/1997 and 320485/1997. The method employing the intermediate electrode is useful in enlarging the lens diameter, but since the required number of components increases, there occur problems, such as the occurrence of astigmatism in an electron lens and the variability of focus or convergence characteristics due to variability in the assembly of components or variability in the assembly of electron guns.
An object of the present invention is to provide a large-diameter electron gun which is reduced in astigmatism and in the variability of focus or convergence characteristics. To this end, the present invention provides a construction which includes a focus electrode having a plate electrode in its inside, an anode electrode having a plate electrode in its inside and an intermediate electrode arranged between the focus electrode and the anode electrode. The focus electrode and the anode electrode are cylindrical electrodes, and the plate electrodes are secured to the inner walls of the respective cylindrical electrodes. The intermediate electrode is made of two cylindrical electrodes and a middle plate electrode, and the middle plate electrode is sandwiched between the two cylindrical electrodes. In accordance with this construction, it is possible to obtain a large-diameter electron gun which is unsusceptible to the influence of assembly and component accuracy and is reduced in the variability of its characteristics. Another object of the present invention is to decrease aberration to a further extent by specifying the ratio of the dimension of the intermediate electrode measured in a direction perpendicular to an inline direction to the length of the intermediate electrode measured in the axial direction of the electron gun. A further object of the present invention is to provide a large-diameter electron gun which is reduced in the variability of its focus characteristic by reducing the potential of the intermediate electrode to 52-58% of an anode potential.